如何確保參數構造函數失敗時不調用默認構造函數？

Question

這是代碼。我必須使用Date類並將其擴展爲創建ExtendedDate。無論如何，我不應該改變Date類。我非常感謝您提供的任何幫助。我茫然不知該如何解決這個

public static void main(String[] args) { 
    /* Trying to create a date with month = 3, date = 40, year = 2010. Objective to is throw an error/exception - "Date can't be created" */ 
    ExtendedDate Dt1 = new ExtendedDate(03,40,2010); 
    System.out.println(Dt1.getDay()); 
    //I don't want this statement to be executed because 40 is not valid. But it's printing "1" which is the default value for the default constructor 
} 

class ExtendedDate extends Date { 
    // Default constructor 
    // Data members are set according to super's defaults 
    ExtendedDate() { 
     super(); 
    } 

    // Constructor that accepts parameters 
    public ExtendedDate(int month, int day, int year) { 
     setDate(month, day, year); 
    } 

    @Override 
    public void setDate(int monthInt, int dayInt, int yearInt) { 
     if (isValidDate(monthInt, dayInt, yearInt)) 
     //isValidDate code is working perfectly fine. 
     { 
      super.setDate(monthInt, dayInt, yearInt); 
     } 
     else { 
      System.out.println("Wrong Date"); 
     } 
    } 

這裏是Date類

public class Date { 

    private int month; // instance variable for value of the date’s month 

    private int day; // instance variable for value of the date’s day 

    private int year; // instance variable for the value of the dates 

    // Default constructor: set the instance variables to default values: month = 1; day = 1; year = 1900; 
    public Date() { 
     month = 1; 
     day = 1; 
     year = 1900; 
    } 

    // Constructor to set the date 
    // The instance variables month, day, and year are set according to received parameters. 

    public Date(int month, int day, int year) { 
     this.month = month; 
     this.day = day; 
     this.year = year; 
    } 

    public void setDate(int month, int day, int year) 
    { 
     this.month = month; 
     this.day = day; 
     this.year = year; 
    } 

    public int getMonth() 
    { 
     return month; 
    } 

    public int getDay() { 
     return day; 
    } 

    public int getYear() { 
     return year; 
    } 

Answer 1

當一些參數是無效的，你應該拋出IllegalArgumentException：

@Override 
public void setDate(int month, int day, int year) { 
    if (isValidDate(month, day, year)) { 
     super.setDate(month, day, year); 
    } 
    else { 
     throw new IllegalArgumentException("Invalid date"); 
    } 
} 

瞭解更多例外情況在the Java tutorial。

請注意，從構造函數調用可重寫的方法是一種不好的做法。您應該直接從構造函數調用isValidDate()（假設此方法是私有的或最終的）。